1. Field of the Invention
This invention relates to an image forming method by electrophotography, electrostatic recording or electrostatic printing, and to an image forming apparatus for carrying out the above method. The present invention is also directed to an electrophotographic toner for use in the above method and apparatus.
2. Discussion of the Prior Art
Various electrophotographic image forming methods have been disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications Nos. 42-23910 and 43-24748. The methods typically include the following processes: (a) a surface of a photoconductor is charged (charging process); (b) the charged surface is exposed to light to form an electrostatic latent image thereon (latent image forming process); (c) the latent image is developed with an electrophotographic toner to form a toner image on the photoconductor (developing process); (d) the toner image is transferred directly or indirectly through an intermediate transfer member onto a transfer member such as paper (transferring process); and (e) the toner image is fixed upon application of heat, pressure, solvent vapor, or combination thereof (fixing process). A cleaning process is generally adopted to remove toner remaining on the photoconductor after the image transferring process.
As the method for developing electrostatic latent images, there are known a magnetic brush method (U.S. Pat. No. 2,874,063), a cascade method (U.S. Pat. No. 2,618,552), a powder cloud method (U.S. Pat. No. 2,221,776) and a method using a conductive magnetic toner (U.S. Pat. No. 3,909,258). A toner for use in these method generally includes a colorant, such as a pigment or a dye, and a binder resin. A magnetic particle such as magnetite is incorporated in the toner for forming a magnetic toner. The toner may be used as such as a single component developer or may be used in conjunction with a carrier, such as glass beads or iron powder, as a two-component developer.
In recent years, the above-mentioned image forming method is utilized not only in copying machines but also in output devices for computers and fixation of microfilms. Thus, there are increasing demands for image forming methods which can simultaneously satisfy many requirements such as a small size, a low cost, a light-weight, low energy consumption, maintenance-freedom and personal use. Accordingly, a toner for use in such image forming methods must meet with various performances. For example, in addition to safety, toner must not generate odors during fixation. While the problem of generation of odors may be reduced by mounting a filter to image forming apparatuses, it is necessary to periodically replace the filter with new one.
Generation of ozone during charging process is also one of the problems involved in the image forming method. Thus, a contact-type charging method using a charging roller is being substituted for a corona discharge method. While the contact-type charging method is free of ozone generation, a toner which has remained unremoved from a photoconductor is apt to deposit to a surface of the charging roller so that charging failure is caused. Such toner deposition is accelerated when the toner having a small particle diameter is used for obtaining fine, clear images.
To cope with the toner deposition problem in the charging roller, Japanese patent No. 2814211 proposes a toner having a benzaldehyde content of 0.005% by weight and containing a binder resin having such a molecular weight distribution that a Mw/Mn (weight average molecular weight/number average molecular weight) ratio is 5 or more and that 10-50% by weight of the binder resin has a molecular weight of 10,000 or less. The known toner, however, is still not fully satisfactory with respect to the prevention of fouling of the charging roller.
It is, therefore, an object of the present invention to provide an image forming method in which charging of a photoconductor is carried out using a charging roller of a contact type and in which toner deposition on the charging roller is prevented.
Another object of the present invention is to provide an image forming apparatus which has a charging roller for charging a photoconductor and in which toner deposition on the charging roller is prevented.
It is a further object of the present invention to provide a toner which is used in an image forming method using a charging roller of a contact type and which is free of a problem of toner deposition on the charging roller.
In accomplishing the foregoing object, there is provided in accordance with one aspect of the present invention an image forming method including charging a surface of a photoconductor by contact with a contact-type charging roller, exposing imagewise the charged surface to form an electrostatic latent image thereon, developing the latent image with a toner, transferring the developed image to a transfer member, and fixing the transferred image,
wherein the charging roller has a surface roughness of 2-40 xcexcm, and
wherein the toner comprises a colorant, and a binder resin comprising a tetrahydrofuran-soluble component having such a molecular weight distribution that at least one peak having a half width not greater than 15,000 is present between 1,000 and 10,000 when measured by gel permeation chromatography.
In another aspect, the present invention provides an image forming apparatus comprising a photoconductor, a charging roller having a surface roughness of 2-40 xcexcm and disposed for contacting with a surface of the photoconductor to charge the surface, exposing means for exposing imagewise the charged surface of the photoconductor to form an electrostatic latent image thereon, a developing device containing a toner for developing the latent image and to form a toner image on the photoconductor, means for transferring the toner image from the photoconductor to a transfer member, and a fixing device for fixing the transferred image to the transfer member, wherein the toner comprises a colorant, and a binder resin comprising a tetrahydrofuran-soluble component having such a molecular weight distribution that at least one peak having a half width not greater than 15,000 is present between 1,000 and 10,000 when measured by gel permeation chromatography.
The present invention also provides a toner for use in an image forming method which comprises charging a surface of a photoconductor by contact with a contact-type charging roller having a surface roughness of 2-40 xcexcm, exposing imagewise the charged surface to form an electrostatic latent image thereon, developing the latent image with the toner, transferring the developed image to a transfer member, and fixing the transferred image,
the toner comprising a colorant, and a binder resin comprising a tetrahydrofuran-soluble component having such a molecular weight distribution that at least one peak having a half width not greater than 15,000 is present between 1,000 and 10,000 when measured by gel permeation chromatography.